1. Field of the Invention
The present invention relates to a method for making a heat-sensitive, negative-working lithographic printing plate precursor.
2. Description of the Related Art
Lithographic printing presses use a so-called printing master such as a printing plate which is mounted on a cylinder of the printing press. The master carries a lithographic image on its surface and a print is obtained by applying ink to the image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional, so-called “wet” lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e., ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e., water-accepting, ink-repelling) areas. In so-called driographic printing, the lithographic image consists of ink-accepting and ink-adhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.
Printing masters are generally obtained by the image-wise exposure and processing of an imaging material called a plate precursor. In addition to the well-known photosensitive, so-called pre-sensitized plates, which are suitable for UV contact exposure through a film mask, heat-sensitive printing plate precursors have also become very popular in the late 1990s. Such thermal materials offer the advantage of daylight stability and are especially used in the so-called computer-to-plate method wherein the plate precursor is directly exposed, i.e., without the use of a film mask. The material is exposed to heat or to infrared light and the generated heat triggers a (physico-) chemical process, such as ablation, polymerization, insolubilization by crosslinking of a polymer, heat-induced solubilization, or by particle coagulation of a thermoplastic polymer latex.
The most popular thermal plates form an image by a heat-induced solubility difference in an alkaline developer between exposed and non-exposed areas of the coating. The coating typically includes an oleophilic binder, e.g., a phenolic resin, of which the rate of dissolution in the developer is either reduced (negative working) or increased (positive working), by the image-wise exposure. During processing, the solubility differential leads to the removal of the non-image (non-printing) areas of the coating, thereby revealing the hydrophilic support, while the image (printing) areas of the coating remain on the support. Typical examples of such plates are described in, e.g., EP-A 625728, EP-A 823327, EP-A 825927, EP-A 864420, EP-A 894622, and EP-A 901902. Negative working embodiments of such thermal materials often require a pre-heat step between exposure and development as described in, e.g., EP-A 625,728.
Negative working plate precursors which do not require a pre-heat step may contain an image-recording layer that works by heat-induced particle coalescence of a thermoplastic polymer latex, as described in, e.g., EP-A 770 494, EP-A 770 495, EP-A 770 496, and EP-A 770 497. These patents disclose a method for making a lithographic printing plate including the steps of (1) image-wise exposing an imaging element including hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder and a compound capable of converting light into heat, (2) and developing the image-wise exposed element by applying fountain solution and/or ink.
U.S. 2002/0009574, U.S. 2002/0051931, and U.S. 2004/0081913 disclose a printing plate precursor including a hydrophilic layer on a metal or flexible support and an imaging layer including heat-fusible particles dispersed in a hydrophilic binder, wherein the hydrophilic layer includes colloidal silica as a porosity providing material.
EP 849 090 discloses an imaging element including on a flexible support (i) an ink repellant layer containing a crosslinked hydrophilic binder, (ii) a thermosensitive layer including hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder, and (iii) an outermost layer including a hydrophilic binder and optionally colloidal silica.
U.S. 2002/0051931 discloses a printing plate precursor including an imaging layer including latex particles which may also contain alkaline colloidal silica.
Research Disclosure No. 33303 dated January, 1992 discloses a heat recording material including on a support a hydrophilic layer containing a hydrophilic binder, a tetraalkyl orthosilicate crosslinking agent, and hydrophobic thermoplastic polymer particles. The hydrophilic layer may further include colloidal silica for improving the mechanical strength and porosity of the layer.
JP 2004/195724 discloses a lithographic printing plate precursor with an imaging layer including on a flexible support an image-forming layer including latex particles dispersed in a hydrophilic binder and colloidal silica.
Plate precursors that work according to the mechanism of heat-induced latex coalescence preferably have a high sensitivity enabling exposure at a low energy density and good clean-out characteristics, i.e., complete removal of unexposed areas during development. However, often high sensitivity printing plates which only require a low energy density to induce coalescence of the latex particles show an insufficient clean-out during development resulting in toning (i.e., ink acceptance in the non-image areas). Attempts in the prior art for improving clean-out behavior of latex based printing plates usually at the same time reduce the sensitivity of the plate.